Continental Basalts And Mantle Xenoliths

Author : Shelby Bowden
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (144 download)

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Download or read book Geochemical Evolution of the Arabian Lithosphere and Genesis of Continental Basalts written by Shelby Bowden and published by . This book was released on 2024 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Arabian Peninsula is the ideal location to study continental basalt volcanism as nearly 6% of the total land area is covered by km-thick primitive basalt flows. The petrogenesis of continental basalts is a poorly understood phenomenon which can be better constrained through a geochemical and geophysical study of the Arabian basalts and their xenolith cargo. Precise 40Ar/39Ar dating shows that Arabian volcanic ages are bimodally distributed: tholeiitic basalts formed thick volcanic plateaus roughly parallel to the Red Sea from the Oligocene through mid-Miocene; after a several million-year volcanic hiatus a second, alkaline volcanic period began in the late-Miocene following the suturing of Eurasia and Arabia and has continued to the present. We integrate new petrographic and geochemical data on basalts and xenoliths from the two largest and longest-lived basalt fields in the Middle East, Harrats Ash Shaam and Uwayrid, with existing data from harrats across the Arabian Peninsula. Geochemical data suggest the early Oligo-Miocene volcanic period was driven by diffuse rifting and adiabatic decompression melting associated with the opening of the Red Sea. Volcanism ceased as rifting along the Red Sea axis slowed during collision with Eurasia. Late-Miocene -- present volcanism is driven by fracturing and subsequent foundering of the lithosphere from faulting associated with the Dead Sea Fault system. This work highlights the dependence of melt generation beneath a continent on far field tectonic stresses. The physical and chemical state of the lithospheric mantle beneath Arabia plays a first order role in defining the ability of melts to form beneath the continent. Mantle xenoliths provide direct evidence of a lithosphere defined by a highly fusible mineralogy. Growth of metasomatic phases clinopyroxene, phlogopite, amphibole, ilmenite, rutile, and enrichment in volatiles has both lowered the melting temperature and increased the density of the lithospheric mantle over that of the asthenosphere, decreasing long-term lithospheric stability. These metasomatic phases are the result of 1 Ga of chemical modification related to Pan African Orogeny subduction that introduced melts, sediments, fluids, and carbon into the lithospheric mantle. Melting of the metasomatic assemblage in xenoliths produces basalts with lower 176Hf/177Hf and higher 208Pb/204Pb and 207Pb/204Pb values than the mantle array, which is consistent with enrichment of the source domains with contributions from slab-derived fluids and sediments. Fractionation of 87Sr/86Sr and enrichment of LREE over HFSE require melt interaction with a carbonate melt metasomatic agent released during Quaternary faulting. 3He/4He ratios in both basalts and xenoliths do not require material contribution from a deep-seated mantle plume but are consistent with contributions from lithospheric and perhaps asthenospheric mantle sources. These results highlight the interplay between mantle source chemistry and tectonics in producing melt beneath continents.